1. Field of the Invention
The present invention relates to a displacement detecting apparatus capable of detecting a positional fluctuation of an object in a non-contact manner, where the positional displacement is minute displacement in the order of nanometers, and an information recording apparatus using such a displacement detecting apparatus.
2. Related Background Art
FIG. 1A of the accompanying drawings shows a perspective view of an information recording reproducing apparatus according to the prior art. The apparatus is comprised of a hard disc drive 1 for writing a servo track signal from a signal generator (SG) 48 into a hard disc, and a rotary positioner system 2 for effecting highly accurate rotary positioning. The hard disc drive 1 comprises a disc-shaped hard disc 3, a slider 4 having a magnetic head at its tip end, a magnetic head arm 5, a voice coil motor 6, a voice coil motor driver 7, a spindle 8, etc. Also, the rotary positioner system 2 comprises a push rod 9, a push rod arm 10, a positioning control motor 11, a rotary encoder 12 for detecting the amount of rotation of the rotary shaft of the control motor 11, a signal processor 13 for analyzing the detection output from the rotary encoder 12 and sending a positioning command signal to the servo track signal writing position of the magnetic head, a control motor driver 14 for driving the control motor 11 by the command signal of the signal processor 13, etc.
With such a construction, the writing and reading of magnetic information are effected on any track on the surface of the hard disc 3 being rotated at a high speed, by the arcuately operating magnetic head arm 5 through the magnetic head. At this time, in order to effect highly accurate positioning, the cylindrical surface of the push rod 9 is pushed against the side of the magnetic head arm 5, and the push rod arm 10 is rotated by the control motor 11 while feedback control is effected by the system of the rotary encoder 12, the signal processor 13 and the control motor driver 14, and positioning is effected while the magnetic head arm 5 is sequentially finely fed through the intermediary of the push rod 9. At this time, in order to effect contact reliably, usually some electric current is supplied to the voice coil motor 6 and pushing is also effected against the push rod 9 from the magnetic head arm 5 side.
FIG. 1B of the accompanying drawings shows a perspective view of another highly accurate positioning apparatus. This detecting apparatus is comprised of a laser source 15, mirrors 16, 17, a beam splitter 18, a retro-reflector 19, like a corner cube provided on a magnetic head arm 5, and a light receiving element 20. Movement of the magnetic head is measured with high accuracy not by the magnetic head arm 5 being mechanically pushed, but by optical means.
In this apparatus, by the utilization of a Michelson-type interferometer comprising the laser source 15, the mirrors 16, 17, the beam splitter 18 and the retro-reflector 19, the interference light of two light beams passed from the retro-reflector 19 via the mirror 16 and the mirror 17 is detected by the light receiving element 20, thereby to obtain positional information of the magnetic head arm 5. On the basis of the obtained detection signal, a signal processor 13 issues a command, and an electric current to be supplied to a voice coil motor 6 is controlled by a voice coil motor driver 7, thereby to directly move the magnetic head arm 5 and effect appropriate control.
FIG. 1C of the accompanying drawings shows a perspective view of the optical system of an optical-type sensor unit 20 according to the prior art, and in the optical-type sensor unit 20, there are successively arranged a multimode laser diode light source 21, a collimator lens 22, a non-polarizing beam splitter 23, and a probe-shaped polarizing prism 24 having a polarizing beam splitter surface 24a and a reference reflecting mirror surface 24b on which reflecting evaporated film is formed. In the reflecting direction of the non-polarizing beam splitter 23, there are arranged a quarter wavelength plate 25, a beam diameter limiting opening plate 26, a beam amplitude dividing diffraction grating 27 having staggered grating structure, polarizing plate analyzers 28a to 28d disposed with their polarization azimuths deviated by 45xc2x0 from one another, and light receiving elements 29a to 29d. 
With such a construction, divergent light from the multimode laser diode light source 21 is made into a loosely condensed light beam L by the collimator lens 22, and is transmitted through the non-polarizing beam splitter 23 and then passes through the probe-shaped polarizing prism 24, and is divided into polarized components in the polarizing beam splitter surface 24a. An S-polarized light beam reflected by the polarizing surface 24a emerges from the end surface of the probe-shaped polarizing prism 24 and is condensed near the beam waist of the measuring surface 5a of the magnetic head arm 5, and the reflected light thereof becomes a divergent spherical wave and passes along the original optical path and returns to the probe-shaped polarizing prism 24. On the other hand, a P-polarized light beam transmitted through the polarizing surface 24a is condensed at a position deviating from the beam waist on the reference reflecting mirror surface 24b in the end portion, and the reflected light thereof passes along the original path and likewise returns to the probe-shaped polarizing prism 24.
These two light beams are re-combined on the polarizing surface 24a of the probe-shaped polarizing prism 24 and become linearly polarized light beams orthogonal to each other, and do not directly interfere with each other and become bright and dark signals, but yet when these two light beams are reflected in the non-polarizing beam splitter 23 and transmitted through the quarter wavelength plate 25, the linearly polarized light beams orthogonal to each other are converted into oppositely circularly polarized light beams, and these two light beams have their vibration surfaces vector-combined and are re-converted into a linearly polarized light beam rotated by the fluctuation of the phase difference therebetween.
This rotated linearly polarized light beam is amplitude-divided into four light beams by the phase diffraction grating 27, and these four divisional light beams are transmitted through the polarizing plate analyzers 28a to 28d, whereby they are converted into interference light beams in which the timing of light and darkness shifts by 90xc2x0 each in terms of phase, and are received by the respective light receiving elements 29a to 29d. On the basis of the light reception signals of these light receiving elements 29a to 29d, a minute fluctuation of the position of the measuring surface 5a of the magnetic head arm 5 is detected with high accuracy of 1 nm or less.
In the above-described rotary positioner system 2 of FIG. 1A, however, vibration due to rotation or the like of the hard disc 3 is transmitted to the magnetic head arm 5, and is further transmitted to the control motor 11 through the cylindrical surface of the push rod 9. Therefore, highly accurate positioning is hindered and the capability of writing information such as a high-density servo track signal is reduced. For this reason, as a method of detecting minute displacement, there is known an electrostatic capacity sensor or the like utilizing impedance, e.g. electrostatic capacity, between the measuring surface 5a of the magnetic head arm 5 and the push rod 9 of the measuring probe. However, in this case there is a problem in that, if the area of the measuring surface 5a is small, the measuring resolving power will be reduced and the output will drift.
Also, in the above-described optical positioning apparatus of FIG. 1B, it is necessary to place the retro-reflector 19 like a corner cube on the magnetic head arm 5, and it requires much care to secure the space therefor and mount and dismount the retro-reflector. There also is a problem in that a control characteristic due to bulkiness and increased weight is aggravated, and is affected by an environmental fluctuation such as the fluctuation of air.
The optical type sensor unit of FIG. 1C is a useful one which has solved the above-noted problems peculiar to the prior art. In this unit, the sensor probe comprising the polarizing prism 24 is small and, therefore, the detecting position range thereof is as small as 100, xcexcm or less. Further, the set position of the sensor probe is proximate to the measuring surface 5a of the magnetic head arm 5 and, therefore, it is necessary to adjust the direction and position of the sensor probe during the setting thereof, to look for an appropriate signal location, and to set the sensor probe so that the level of the signal may become greatest.
Also, when the direction of the measuring surface 5a is predetermined, there is adopted a method of supporting the sensor probe on an X stage or the like with its direction made substantially perpendicular to the measuring surface 5a, setting it in a direction to take in reflected light, approximating the sensor probe to the measuring surface 5a and looking for an appropriate signal location, and setting the sensor probe at the center of the range thereof. In this case, the detecting position range of the sensor probe is small and therefore, when the signal is to be caught to thereby determine the center of the signal, the sensor probe is gradually reciprocated several times. Outside the detecting position range, there is no signal from this sensor probe, and even within the detecting position range, the direction of the light condensing position cannot be known. Consequently, for example, to automatize measurement, a further improvement becomes necessary.
The present invention has been made in view of what has been described above, and an object thereof is to provide a displacement detecting apparatus capable of discriminating the position of the surface of reflecting means in which the surface of the reflecting means and polarizing separating means are in a proper positional relation.
Another object of the present invention is to provide a displacement detecting apparatus which can detect the position of an object with high reliability without providing a discrete member on the object side, and makes positioning of high accuracy and high resolving power possible.
Still another object of the present invention is to provide an information recording apparatus capable of writing a servo track signal highly accurately into a hard disc.